During sustained activity, an individual's feet are subjected to large, repetitious, ground reaction or impact forces generated in a gait cycle. The ground reaction forces associated with foot strike while walking are typically between one and one-and-one-half an athlete's body weight. Runners impact the ground with vertical forces as high as three to four times their body weight, depending upon their speed. In more dynamic activities, such as aerobics and basketball, impact forces as high as five to six times an athlete's body weight have been recorded.
The human body attenuates ground reaction forces through a complex 3-dimensional motion of the foot at the subtalar, metatarsal, and other joint areas. However, such natural biomechanics often cannot attenuate or dissipate impact forces sufficiently to prevent injury. Breakdown in the joints often results from insufficient attenuation or dissipation of ground reaction forces at the regions upon which the impact is focused. Those areas are concentrated in the heel strike and metatarsal regions of the foot.
Many components and materials which provide cushioning that attenuates and dissipates ground reaction forces are known. Prior art shoes have long incorporated a mid-sole composed of ethylvinylacetate ("EVA"), a closed cell foam material. EVA is a lightweight and stable foam that possesses viscous and elastic qualities. In addition, the density or durometer of EVA can be altered by adjusting the manufacturing technique.
Unfortunately, closed cell foam cushioning materials add stiffness to a shoe. As the thickness of the cushioning material is increased, the shoe loses the flexibility which is required to accommodate a natural running or walking gait cycle, and the shoe becomes uncomfortable for the wearer. Injury can sometimes result. Also, the desirable flexibility properties of a shoe will vary with its intended use. Shoes intended to be used on hard smooth surfaces (such as paved roads) may require flexibility both along the long or longitudinal axis of the shoe, as well as along the transverse that axis. Flexibility perpendicular to the long axis of the shoe can be undesirable in some shoes, such as off-pad or trail shoes.
Thus, there is a need for a lightweight shoe with enhanced cushioning properties and which has appropriate flexibility properties.